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The author is expecting her first child with husband Chris Pratt, and it seems Arnie can't wait to shower his future grandchild with love.

Speaking about the pregnancy news on The Tonight Show Starring Jimmy Fallon, Arnie gushed about his excitement.

And he suggested that Katherine could be further along in her pregnancy than fans realised, saying she's due to give birth in the summer.

He said: "Can you believe that? That is really exciting news that Katherine now is pregnant and having a baby.

Arnold with daughter Katherine (Image: Getty Images North America)

"I don’t know when exactly. Sometime this summer. Inevitably, I’m really looking forward to playing around with whatever it is, she or he, and having some fun."

Terminator star Arnie joked that there will be "a lot of power" in his grandchild thanks to its inherited genes from himself, Katherine, 30, and Guardians of the Galaxy star Chris, 40.

He said: "I think this is kind of a unique experience... just thinking about the gene pool.

Katherine and Chris are expecting their first baby together (Image: Getty Images for Disney)

"Think about this for a second.... you can do Kennedy, Schwarzenegger and Pratt. We can do anything. We can go and solve the Cuban Missile Crisis, we can go and kill predators with our bare hands and we go and train dinosaurs."

But he said the one thing he doesn't want the child to inherit is his famous accent.

"That's what we don't want," he joked.

They got married last year (Image: FilmMagic)

News broke last month that Chris and Katherine are expecting their first child together.

Chris is already dad to Jack, seven, who he shares with ex-wife Anna Faris.

Chris and Katherine tied the knot in June 2019, and seven months later he opened up about their plans to start a family.

He told Entertainment Tonight in January that he wanted "lots of kids".

He said: "I always want to be that guy that works to live, not lives to work. I think [I want to spend] just a lot of time at the farm and a lot of time catching a lot of fish and seeing lots of sunsets."